Comments on the dynamical effects of radiative viscosity

In this paper we make estimates of the relative sizes of terms in the diffusion-limit radiation stress-energy tensor, and in the dynamical equations for a radiating fluid, with particular emphasis on the effects of radiative viscosity. By a comparison of the results of radiation diffusion theory with comoving-frame versions of Eckart's equations for a relativistic nonideal fluid, we are able to show a complete parallelism between the radiation and material terms. We further show the Castor's comoving-frame radiation energy and momentum equations, augmented by the acceleration terms discussed by Buchler, provide a completely consistent treatment of all effects associated with radiation viscosity and with the equivalent inertia of radiation enthalpy and the radiant energy flux. From this analysis we further obtain a definite interpretation of the physical significance of the acceleration terms in the comoving-frame radiation equations